Alternating current machine with increased torque above and below rated speed for hybrid electric propulsion systems

ABSTRACT

The machine in accordance with the present disclosure is an AC machine whose pole numbers can be switched (from pole p 1  to pole p 2 ), and whose number of series turns per phase N can be switched say from N 0 =N rated  to N 1 =N 0 /2. Furthermore, it employs an inverter so that the frequency can be changed from a low value (e.g., 5 Hz) to a high value (e.g., 200 Hz). Due to the combination of pole number and number of series turns switching/reconfiguration, a high torque at low speed (e.g., 0 rpm) and a high torque at high speed (e.g., 5,000 rpm) can be achieved, making mechanical gears obsolete. In addition, the output power of the motor can be increased at high speed in direct proportion to the speed increase.

RELATED APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalPatent Application No. 61/135,788, filed Jul. 24, 2008 entitled“Alternating Current Machines With Increased Torque (Larger Than RatedTorque) Above And Below Rated Speed For Hybrid/Electric PropulsionApplications”.

FIELD OF THE DISCLOSURE

The present disclosure relates to electric motors and generators. Inparticular, it relates to systems and methods for operating suchmachines above, at, and below rated machine speed and torque, and foroperating at multiples of rated speed, at rated torque, by compensatingfor flux weakening.

BACKGROUND OF THE DISCLOSURE

Electric drives require for starting (below rated speed), a torque whichis a multiple of the rated torque, and for higher speeds (above ratedspeed), an increased torque. While the first one is required toguarantee a smooth start-up, the latter is desirable to warrantsufficient acceleration torque to improve dynamic performance. It iswell known that speed control—based on (V/f) control—results in adecreasing torque above rated operation (T_(rated), n_(m) _(—)_(rated)).

Rated speed is the maximum speed that an electric motor can run at,while also running at a rated torque, continuously for a significantperiod of time without damaging the motor. Rated torque is the maximumtorque that an electric motor can produce continuously for a significantperiod of time without damaging the motor.

SUMMARY OF THE DISCLOSURE

The present disclosure relates to systems, method, and apparatus forachieving greater than rated operating conditions (e.g., speed, torque,power) in a multiphase electric machine. The machine achieves greaterthan rated operating conditions by changing the number of poles and byswitching the number of series turns (or inductance) of each phase belt.

In one aspect, a multiphase inductance-changing and pole-changingelectric machine operable at above rated parameters may have a statorhaving a plurality of phase belts, wherein each phase belt comprises twoor more coils, a rotor driven by currents in the stator, and a controlsystem connected to the plurality of phase belts. Each of the two ormore coils in a phase belt are initially connected in series. Thecontrol system may be configured to apply an alternating current to theplurality of phase belts at a first time and at a frequency equal to afirst frequency. The first frequency may be below a rated frequency (thefrequency at which the machine can operate continuously withoutoverheating or experiencing any permanent damage). The control systemmay be configured to increase the frequency to a second frequency duringa time period spanning the first time and a second time. The controlsystem may be configured to decrease the number of poles at a thirdtime. The control system may be configured to increase the frequency toa third frequency during a time period spanning a fourth and a fifthtime. The control system may be configured to decrease the inductance ofthe plurality of phase belts at a sixth time. The control system may beconfigured to increase the frequency to a fourth frequency during a timeperiod spanning a seventh time and an eighth time.

In one aspect, decreasing the number of poles may be accomplished byswitching the series connections between phase belts to parallelconnections. In one aspect, the time period spanning the fourth andfifth time periods may be minimized without saturating the machine. Inone aspect, decreasing the inductance of the phase belts may beaccomplished by switching the series connection(s) between two or morecoils in each phase belt to parallel connections. In one aspect, thephase belts may be initially connected in a delta configuration. Todecrease the number of poles, the delta configuration may be switched toa double wye configuration. In one aspect, the phase belts may initiallybe connected in a double wye configuration. To decrease the number ofpoles, the double wye configuration may be switched to a deltaconfiguration. In one aspect, the phase belts may initially be connectedin a wye configuration. To decrease the number of poles, the wyeconfiguration may be switched to a double wye configuration. In oneaspect, the second frequency may equal the rated frequency. In oneaspect, the second time may equal the third time. In one aspect thefifth time may equal the sixth time.

In one aspect, a multiphase inductance-changing and pole-changingelectric machine operable at above rated parameters may have a statorhaving a plurality of phase belts, wherein each phase belt comprises twoor more coils, a rotor driven by currents in the stator, and a controlsystem connected to the plurality of phase belts. Each of the two ormore coils in a phase belt may initially be connected in series. Thecontrol system may be configured to increase the frequency of analternating current applied to the plurality of phase belts during afirst time period. The control system may be configured to decrease anumber of poles in the stator at a second time. The control system maybe configured to further increase the frequency of the alternatingcurrent applied to the plurality of phase belts during a second timeperiod. The control system may be configured to decrease an inductanceof the plurality of phase belts at a third time. The control system maybe configured to further increase the frequency of the alternatingcurrent applied to the plurality of phase belts during a third timeperiod.

In one aspect, the second time period may follow the second time(meaning it does not start at the second time, but starts after thesecond time). In one aspect, the third time period may follow the thirdtime.

In another aspect, a method is disclosed for operating an alternatingcurrent machine at above rated parameters. The method may includeincreasing the frequency of an alternating current applied to aplurality of stator phase belts during a first time period. The methodmay include decreasing the number of poles in the stator at a secondtime. The method may include further increasing the frequency of thealternating current applied to the stator phase belts during a secondtime period. The method may include decreasing the inductance of theplurality of stator phase belts at a third time. The method may includefurther increasing the frequency of the alternating current applied tothe plurality of stator phase belts during a third time period. In oneaspect, the second time period may follow the second time. In oneaspect, the third time period may follow the third time.

A machine and method for operating an alternating current machine belowand above rated speed and torque is disclosed. Such a drive using(V·p/f·N) control—as taught by this disclosure, where V≦V_(rated) andN≦N_(rated)—with increased torque and speed operation without increasingthe machine size or weight but increasing the output power rating (e.g.,by a factor of two) will find applications in the area of hybrid andelectric drives for automobiles, military vehicles, and wind powerplants, just to name a few. The principle for this extended operatingrange is based on 1) pole-changing techniques and 2) changing/switchingthe number of series turns per phase N of an alternating current/voltagemachine via electronic switches or relays and the use of aninverter/rectifier supplying/absorbing power to/from the electricmachine when operated as a motor/generator.

The machine in accordance with the present disclosure is an AC machinewhose pole numbers can be switched (e.g., from pole p₁ to pole p₂), andwhose number of series turns per phase belt N can be switched say fromN₀=N_(rated) to N₁=N₀/2. Furthermore, it employs an inverter so that thefrequency can be changed from a low value (e.g., 5 Hz) to a high value(e.g., 200 Hz). The high number of poles at low speed (e.g., near 0 rpm)provides high torque during starting. In order to reach higher speeds,the number of poles can be decreased and, shortly thereafter (e.g., onesecond), the number of windings per pole can be decreased (increasingflux density and compensating for flux weakening). This decrease in thenumber of turns per pole provides torque that is greater than ispossible using conventional (V/f) control at high speed (e.g., 5,000rpm). In addition, the output power of the motor can be increased (e.g.,by a factor of two) at high speed in direct proportion to the speedincrease. This combination of high starting pole number, decreased polenumber, and then decreased number of windings per pole obviates the needfor mechanical gears.

The reconfiguration of the stator winding requires significantly morethan 6 switches. During transient operation the flux density within themachine can be increased above its rated value. While traditional (V/f)control can only achieve speeds around three times the rated or basespeed, the (V·p/f·N) control, herein disclosed, can achieve output powerat least six times the rated or base speed. The present disclosure canalso increase output power by at least a factor of two over traditional(V/f) control. The machine as proposed by this disclosure can work bothas a motor and as a generator at variable speed, due to the pole andnumber of turns switching and the inverter supplying currents/voltagesat variable frequency.

The use of pole-changing and number of turns changing supplantmechanical gears, thus requiring less space, causing less loss, andresulting in less weight than systems using mechanical gears.

In one exemplary embodiment, the start-up time from 0 rpm to more than 4times the base speed may be not more than a few seconds. In someembodiments, the inverter input power may be supplied by a battery oranother power source. The efficiency either as a motor or an alternatorin various embodiments may be in the 75-95% range depending upon theoutput power rating of the variable-speed drive in accordance with thisdisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a torque-speed characteristic of a wind turbine T∝n_(m) ².

FIG. 2 shows a torque-speed characteristic of an electric motor atspeeds from zero to three times rated speed (3n_(s)).

FIG. 3 shows a torque-speed characteristic of an electric motor atspeeds from zero to six times rated speed (6n_(s)) and taking advantageof the increased torque resulting from a reduction in the number ofturns (e.g., N_(rated) to N_(rated)/2) at speeds above 2n_(s).

FIG. 4 shows a plot of measured electric motor drive speed (y-axis) interms of time (x-axis) with a pole number change from p₁ to p₂ and fromp₂ to p₁ (one horizontal division corresponds to 200 ms; one verticaldivision corresponds to 800 rpm).

FIG. 5 shows speed-torque characteristics for the startingcharacteristic and characteristics 1-4 corresponding to four operatingregions described in the disclosure.

FIG. 6 shows 3 speed-torque characteristics between the startingcharacteristic and characteristic 1 of FIG. 5.

FIG. 7 shows speed-torque characteristic 1 and 2 of FIG. 5.

FIG. 8 shows 4 speed-torque characteristics between characteristic 2 and3 of FIG. 5.

FIG. 9 shows 3 speed-torque characteristics between characteristic 3 and4 of FIG. 5.

FIG. 10 is an example of a frequency-time diagram showing six operatingregions of the present disclosure.

FIG. 11 is an example of a speed-time diagram showing six operatingregions of the present disclosure.

FIG. 12 illustrates two 3-phase stator winding diagrams for anembodiment of the p₁-pole configuration, delta (Δ), and the p₂-poleconfiguration, double wye (Y).

FIG. 13 illustrates an embodiment of a p₁ pole configuration (N_(rated),8-pole, Δ).

FIG. 14 illustrates an embodiment of a p₂ pole configuration (N_(rated),4-pole, double Y).

FIG. 15 illustrates terminal leads of phase belts for the embodimentsillustrated in FIGS. 13 and 14.

FIG. 16 illustrates a 3-phase current diagram with a rotating time axis.

FIG. 17 illustrates a plot of magneto-motive force (“MMF”) for the FIG.13 winding configuration.

FIG. 18 illustrates a plot of MMF for the FIG. 14 winding configuration.

FIG. 19 illustrates a first view of a first embodiment of a windingconfiguration.

FIG. 20 illustrates a second view of the winding configurationillustrated in FIG. 19.

FIG. 21 illustrates a first view of a second embodiment of a windingconfiguration.

FIG. 22 illustrates a second view of the winding configurationillustrated in FIG. 21.

FIG. 23 illustrates phase A of a third embodiment of a windingconfiguration.

FIG. 24 illustrates phase B of the third embodiment of a windingconfiguration.

FIG. 25 illustrates phase C of the third embodiment of a windingconfiguration.

FIG. 26 is an oscilloscope readout showing current versus time, andcreated by the winding configuration illustrated in FIGS. 21 and 22.

FIG. 27 is an expanded view of the startup region of the oscilloscopereadout of FIG. 26.

FIG. 28 is an oscilloscope readout showing the startup region, and theswitch from N₀ to N₁ occurring many seconds after the switch from p₁ top₂.

FIG. 29 is an oscilloscope readout showing the startup region, and theswitch from N₀ to N₁ occurring around one second after the switch fromp₁ to p₂.

FIG. 30 is an oscilloscope readout showing voltage versus time for agenerator (upper trace) and a rectifier (lower trace), and created bythe winding configuration illustrated in FIGS. 21 and 22.

DETAILED DESCRIPTION

Reference will now be made in detail to the accompanying drawings, whichat least assist in illustrating various pertinent embodiments andaspects of the present disclosure. The presently disclosed electricmachine can be used as a motor fed by an inverter, or generator feedinga rectifier.

For the purposes of this disclosure, an electric machine 100, as shownin FIG. 20, comprises a stator 102 and a rotor 104, where the stator isstationary, and alternating currents (AC) in phase belts of the statordrive the rotor to rotate (in the case of a wind turbine or in a carconfigured to generate energy from braking the rotor turns and generatescurrents in the stator). When an alternating current (AC) is passedthrough the phase belts of the stator, the changing current induces amagnetic field that inductively couples to the rotor. For the purposesof this disclosure, the rotor is a rotating armature of a motor orgenerator. The rotor has either electrically-conductive elements, atleast one permanent magnet, or neither electrically-conductive elementsnor at least one permanent magnet. The stator has multiple poles, eachformed from two or more electromagnets. The magnitude of current passedthrough each electromagnet varies in time so as to create a rotatingmagnetic field. If the rotor has a permanent magnet, then the rotatingmagnetic field drives the permanent magnet of the rotor to rotate andthus rotate a shaft that can be connected to the wheels of an engine orthe blades of a wind turbine, for example. If the rotor has conductiveelements, but no permanent magnet (e.g., squirrel cage configuration),the rotor's conductive elements cut through the magnetic flux generatedby the stator and induce currents in the rotor. These currents generatemagnetic fields that effectively replicate the permanent magnet of thepermanent magnet-style rotor. The rotating magnetic field of the statorthen drives the rotor to rotate and again this rotates a shaft that canbe used to drive an engine (or when used in reverse can be used togenerate electrical power).

For the purposes of this disclosure, a stator is a mechanical deviceconsisting of the stationary part of a motor or generator in or aroundwhich the rotor revolves. The stator includes a plurality of statorslots. Stator slots are radially-oriented cuts or openings in the statorin which portions of the phase belts reside. The pieces of statorbetween stator slots are herein referred to as teeth.

For the purposes of this disclosure, a “phase belt” is one or more turnsof conductive wire that pass through a pair of slots. A phase belt isalso known as an inductor or an induction coil. The ends of the wire ina phase belt, referred to as “leads,” can be connected to the leads ofother phase belts, to a control system, or to both. For instance, onelead of a phase belt may be connected to the control system while theother lead is connected to the lead of another phase belt. The statorteeth are made from magnetically permeable material (e.g., iron orsteel) and thus enhance the magnetic field that is formed when currentpasses through a phase belt.

A phase belt may comprise a single conductive wire or two or moreconductive wires. Each continuous conductive wire will be referred to asa coil. If there are two or more coils in a phase belt, then the two ormore coils can be connected in series or in parallel. When connected inseries any number of coils act as if the phase belt were made of asingle conductive wire. The total inductance of the phase belt,L_(total), is the sum of the inductance of the coils (L₁ L₂, . . .,L_(n)).L _(total) =L ₁ +L ₂ +. . . +L _(n)

Thus the inductance of a phase belt comprising either a plurality ofcoils connected in series or a phase belt comprising a single coil isL_(total).

However, if any two or more of the coils are connected in parallel, thenthe phase belt's total inductance decreases. This effect is the same assumming the total inductance of discrete inductors connected inparallel.

$L_{total} = \frac{1}{\frac{1}{L_{1}} + \frac{1}{L_{2}} + \ldots + \frac{1}{L_{n}}}$

As seen, when coils are connected in parallel, the total inductance forthe phase belt decreases.

Since the inductance of an inductor is proportional to the number ofturns in that inductor, changing the number of turns and changing theinductance of an inductor achieve the same goal—changing the magneticflux that can be produced (or absorbed) by a given inductor. Thus, forthe purposes of this disclosure changing the inductance and changing thenumber of turns will be used interchangeably. For the purposes of thisdisclosure, when reference is made to a change in inductance or thechange in the number of turns, what is meant is that two or more coilsin a phase belt are switched from series connections to parallelconnections (or vice versa). In one embodiment, inductance can bealtered by switching series connections between phase belts, in a givenphase, to parallel connections.

For purposes of this disclosure, a “turn” means a single wrap or loop ofconductive wire. A coil comprises one or more turns.

For the purposes of this disclosure, a “winding” means all phase beltsin an electric machine. Winding will be used to refer to all phase beltsin all phases together. For instance, if there are three phases, andfour phase belts per phase, then the winding includes all twelve phasebelts and all connections between those phase belts.

A magnetic pole is a region at each end of a permanent magnet where theexternal magnetic field is strongest (the strongest magnetic field isactually internal to the permanent magnet). Electromagnets also havepoles. An electromagnet has a similar magnetic field to a permanentmagnet, but is formed by passing current through a plurality ofconductive coils (e.g., a phase belt). For the purposes of thisdisclosure, an electromagnet may comprise one or more phase belts. Forinstance, an electromagnet may be one or more phase belts connected inseries or parallel and positioned in the same two stator slots.Alternatively, an electromagnet may be a plurality of phase beltsconnected in series or parallel and positioned in four or more statorslots. Like the permanent magnet, the strongest field in anelectromagnet is found within the bounds of the coils. The “external”magnetic field is thus completely outside the bounds of the coils.

For purposes of this disclosure, a pole means one end of anelectromagnet or group of phase belts having the same polarity. Eachelectromagnet so formed has a single pole pair (North and South). Forinstance, the North pole of a first phase belt and the South pole of asecond phase belt positioned opposite to the first phase belt in astator form a single pole pair. The number of poles can be changed byswitching the phase belts from series to parallel connections and viceversa. Thus, when referring to pole changes herein, what is meant ischanging the number of pole pairs in the stator configuration beingdescribed.

For the purposes of this disclosure, a control system or controller 106,shown in FIG. 20 may include one or more hardware components or softwarecomponents operating on a computer system. The control system 106 may beconfigured to control a voltage or current source and the switchingnetwork, both of which are connected to the stator phase belts. Thecontrol system controls not only the current and/or voltage applied tothe phase belts, but also the frequency at which the current and/orvoltage is applied to the phase belts. The control system also controlsthe switches which control how the coils are connected to each other andhow the phase belts are connected to each other. In other words, thecontrol system controls the number of poles and the number of turns perphase belt.

To summarize, this disclosure describes an electric machine using alarge number of poles at low speeds to generate a large flux. Once themachine has begun to rotate, or shortly thereafter, the number of polescan be decreased. Once the number of poles have been decreased the speedcan be ramped up further. However, shortly after the number of poleshave been decreased, the number of turns per phase belt can bedecreased. This switch to a lower number of phase belts shouldpreferably occur immediately after the number of poles has been changed.However, to avoid saturating the machine, the number of turns should bedecreased at least one second after the number of poles has beendecreased.

Basic Principle of (V/f·N) and (V·p/f) Control

The induced voltage E of an alternating current electric machine (eithermotor or generator) is related to the rated maximum flux densityB_(max), the rated number of series turns per phase belt N_(rated), theradius of the location of the stator phase belts R, the active (core)machine length L, the frequency of the voltages/currents f, and the polenumber p by:E=4.44·f·B _(max) ·N _(rated)·4·R·L/p.

Rated values are those values at which the machine can continuouslyoperate at without overheating.

For rated induced voltage E_(rated) the frequency f and the maximum fluxdensity assume rated values. If E<E_(rated) then the flux density isless than its rated value, and for E>E_(rated) the flux density will beabove its rated value. The latter case should not be maintained forsustained time periods as overheating can damage the machine. Thefollowing relation describes steady-state operation when E≦E_(rated) andf≦f_(rated):

${\frac{E}{f} = {{4.44 \cdot B_{\max} \cdot N_{rated} \cdot 4 \cdot R \cdot {L/p}} = {constant}}}\;$

For f>f_(rated) the induced voltage can be at the most E=E_(rated) andthe flux density will be less than its rated value, called fluxweakening operation. For operation above rated speed, where f>f_(rated),the induced voltage E can be replaced in the above formulas by theterminal voltage V resulting in (V/f) control. By changing the number ofseries turns per phase N and the pole number p of the machine the fluxweakening can be compensated for and the machine can be operated at upto the rated flux density above rated speed. This results in superiorperformance (e.g., increased torque and increased power).

For example, it is well-known that the torque-speed characteristic ofwind turbines (see FIG. 1) and commonly available variable-speedgenerators employing flux (field) weakening (see FIG. 2) do not matchbecause the torque of a wind turbine is proportional to the square ofthe speed, and the torque of a variable-speed generator is inverselyproportional to the speed in the field-weakening region. The sameapplies to variable-speed drives (motors/generators) of hybrid/electriccars where the critical (maximum) speed is limited by the reduction(from flux weakening) of the developed torque, resulting in less thanthe rated output power. One way to mitigate this mismatch is toelectronically change the number of poles p, or to change the number ofseries turns per phase belt N (e.g., via an application specificintegrated circuit (“ASIC”)). The change in the number of poles p can bedescribed by the following relation:

$\frac{E \cdot p}{f} = {4.44 \cdot B_{\max} \cdot N_{rated} \cdot 4 \cdot R \cdot L}$where B_(max) is the rated maximum flux density at the radius R of themachine, N_(rated) is the rated number of series turns per phase belt,and L is the axial iron-core length of the machine.

Preferably, the maximum flux density B_(max) of the machine at anyoperation (e.g., start-up, variable-speed operation, regenerativebraking) should be within the range 0.3T≦B_(max)≦1.1T. On the one hand,if B_(max) is too small, the torque will be reduced to an unacceptablelow level, the machine operation may be sluggish and the desired speedsand torques might not be obtained. On the other hand, if B_(max) is toolarge, the machine saturates and the losses will become too large.

The change of the number of series turns per phase N can be described bythe following relation:

$\frac{E}{f \cdot N} = {4.44 \cdot B_{\max} \cdot 4 \cdot R \cdot {L/{p.}}}$

One can see that decreasing the number of series turns per phase Nenables a higher frequency without decreasing the flux density (e.g.,rated flux density B_(max)). Hence, decreasing N compensates for fluxweakening (i.e., flux density can remain constant while frequency isincreased past the frequency at which flux weakening would normally setin). The change of p will similarly affect this (E/f) relationship. Forexample, adding this degree of freedom will permit wind turbines tooperate under stalled conditions at all speeds, generating the maximumpossible power at a given speed with no danger of runaway. This willsimplify the mechanics and control of blade-pitch.

Recall that decreasing the number of series turns per phase N isequivalent to decreasing phase belt inductance. In one embodiment, wherea phase belt comprises two or more coils, this can be accomplished bychanging the series connections between the coils to parallelconnections. In another embodiment, this can be accomplished, bychanging the series connections between phase belts, to parallelconnections.

FIG. 3 shows a torque-speed characteristic of an electric motor atspeeds from zero to six times rated speed (6n_(s)) and taking advantageof the increased torque resulting from a reduction in the number ofturns (e.g., N_(rated) to N_(rated)/2) at speeds above 2n_(s). Thenumber of stator turns per phase belt can be denoted as N. An initialnumber of stator turns per phase belt is N₀. In FIG. 3 speed(revolutions per minute (rpm) or rotational velocity) of the machine isplotted on the y-axis and is denoted n_(m). Torque (T) and power (P) areplotted on the x-axis. In the illustrated characteristic, as the machineaccelerates from a speed of zero to the rated speed, n_(s), the torqueis constant and is equal to the rated torque, T_(rat). This avoidsexceeding the rated current. However, when the speed exceeds the ratedspeed, n_(s), the torque begins to decrease as field weakening takeseffect. At twice the rated speed, 2n_(s), the number of stator turns ishalved from N₀ to N₀/2. This decrease in the number of turns increasesthe flux density associated with the phase belts and thus compensatesfor the flux weakening. As a result the torque doubles from T_(rated)/2to T_(rated). As the machine continues to accelerate the torque againdecreases due to flux weakening. However, despite this decrease, thetorque is still greater than it would be were the original number ofturns, N₀, still being used.

Moreover, when torque decreases to a certain point, acceleration is nolonger possible. For instance, at 3n_(s) with the original number ofturns, N₀, the torque goes to T_(rated)/3, and hence the engine cannotaccelerate very well above 3n_(s) because the torque falls belowT_(rated)/3 (although it appears that torque goes to zero at 3n_(s) andagain at 6n_(s), this is just an industry convention, and in reality thehyperbolic curves continue to extend to higher speeds). Thus, an addedadvantage of the increased torque due to reducing the number of turns isthat sufficient torque (e.g., twice the torque) is generated at highspeeds (e.g., above 3n_(s)) enabling the machine to accelerate beyondthe top, or critical, speed (e.g., 3n_(s)) of the machine using theoriginal number of turns N₀ to an increased critical speed (e.g.,6n_(s)).

It should be understood that the values used in FIG. 3 are non-limiting.For instance, the starting torque is not limited to the rated torque,T_(rat). The curvature of the flux weakening regions is alsoillustrative rather than limiting. The number of turns can be decreasedat any point, not just at 2n_(s). However, preferably the number ofturns is decreased at speeds above where flux weakening sets in. Thedecrease in the number of turns also can be any fraction not just onehalf. Finally, while FIG. 3 only shows a characteristic wherein thenumber of turns is decreased a single time, in other embodiments thenumber of turns can be decreased more than one time in order tocompensate for flux weakening.

FIG. 4 shows a plot of measured electric motor drive speed (y-axis) interms of time (x-axis) with a pole number change from p₁ to p₂ at andfrom p₂ to p₁ (one horizontal division corresponds to 200 ms; onevertical division corresponds to 800 rpm). This figure showsexperimental verification that the change in pole numbers does notinterrupt the smooth deceleration (left half of plot) or smoothacceleration (right half of plot). At 0 s the machine is instructed torun at 4000 rpm. At around 20 ms the machine is instructed to run at 600rpm. The actual speed gradually decreases until shortly before 400 mswhere the pole number is switched from p₂ to p₁. The speed smoothlydecreases through this transition point and continues until it bottomsout at 600 rpm. The machine is then instructed to run at 4000 rpm atapproximately 1.02 s. The actual speed gradually increases until around1.5 s where the pole number is changed from p₁ to p₂. The speed smoothlyincreases through this transition point and continues until it tops outat 4000 rpm.

Overall Design Approach

An alternating current (I) at a voltage (V) is passed through the phasebelts at a frequency f. The voltage can be measured between the lines,or terminals, of the machine rather than between one line and ground.Such a measurement is referred to as line-to-line voltage and is labeledV_(line-line). To increase the speed of the machine the frequency f ofthe current or voltage applied to the phase belts is increased. Thisgenerates a torque T and a power P. The result is that the machinerotates at a speed n (also known as rotational velocity or revolutionsper minute (rpm)). The rated value of any of these is the value that themachine can be operated at for extended periods of time (steady-state)without the machine overheating or being permanently damaged.

The machine can be an induction machine, a synchronous machine, apermanent-magnet machine, a switched-reluctance machine, a homopolarmachine, or a brushless DC machine. In one embodiment, the pole numberis changed at low speeds while the number of turns is changed at highspeeds. Increasing the pole number increases torque at low speed (around0 rpm). A decrease in the pole number will result in the machineincreasing speed without an increase in the frequency of the current orvoltage which is applied to the phase belts. It puts the machine into adifferent operating region wherein an increase in the frequency fgenerates more torque than the increase in frequency would generate witha larger number of turns. Decreasing the number of turns increases theflux density and thus the torque.

The rated torque T_(rated) and the base speed n_(base) can be derived asfollows. Without changing the pole number or number of turns the maximumtorque T_(max) for frequencies f≦f_(rated) is written as:

${T_{\max} = {{\pm \frac{3}{2\;\omega_{m\; s}}}\frac{{{\overset{\sim}{E}}_{rated}}^{2}}{X_{r}^{\prime}}}},$where the synchronous mechanical angular velocity is:ω_(ms)=ω/(p/2)=2πf/(p/2),and a rotor leakage reactance can be written as follows:

${X_{r}^{\prime} = {\frac{m \cdot \left( {p/2} \right)}{N_{r}}{\left( {2\;{N_{s} \cdot \varsigma_{s}}} \right)^{2} \cdot 1.58 \cdot f}\frac{1}{p}{\left( {\Lambda_{slotr} + \Lambda_{endr} + \Lambda_{diffr}} \right)\lbrack\Omega\rbrack}}},$where m is the number of stator phases, p is the number of poles, f isthe frequency at which the voltage/current is applied to the turns,N_(s) is the number of turns per phase belt, ζ_(s) is a stator turnsfactor, and N_(r) is the number of rotor phases (bars). Λ_(slotr),Λ_(endr), and Λ_(diffr) are the rotor slot, rotor end region, and rotordifferential permeances of the rotor leakage, respectively. While thedominant part, the slot leakage, is independent of p, the end region anddifferential leakages depend to some extent on p. If this influence isneglected, T_(max) can be written as follows:

${T_{\max} \approx {{\pm \frac{3p}{4\omega}}\frac{{{\overset{\sim}{E}}_{rated}}^{2}}{X_{r}^{\prime}}}},$

In other words, T_(max) is approximately proportional to the number ofpoles p.

The rated torque T_(rated) is obtained from the following relationship:P _(rated) =T _(rated)·ω_(m) =C·D _(i) ² ·L _(i) ·n _(m),where C is the utilization factor of the machine, D_(i) the rotordiameter, and L_(i) the ideal rotor length. C is nearly independent ofthe pole number p. The value n_(m) can be approximated as follows:n _(m) ≈n _(ms)=120·f/p,

Substituting this approximation for n_(m) into the relationship forP_(rated) gives the following relationship:

${P_{rated} \approx {T_{rated} \cdot \left( \frac{2\;\pi\; f}{p/2} \right)}} = {C \cdot D_{i}^{2} \cdot L_{i} \cdot {\left( \frac{120 \cdot f}{p} \right).}}$

As seen, T_(rated) is independent of p, provided the influence of thephase belt design is neglected. However, this does not apply to themaximum torque T_(max). If the influence of the phase belt design is notneglected, then the rated torque T_(rated) will be different for the p₁pole (low-speed) configuration and that of the p₂ pole (high-speed)configuration, as will be discussed below. Neglecting the influence ofthe phase belt design, T_(rated) can be defined as follows:T_(rated)=T_(rated) ^(p1)=T_(rated) ^(p2)andn_(base)=n_(ms) ^(p1)≈n_(m) _(—) _(rated) ^(p1).

Traditional machines can maintain rated torque until voltage reaches itsrated value V_(rated). Above that point the voltage remains constantwhile the frequency increases thus decreasing the flux densityassociated with the phase belts. This type of machine is described ashaving V_(rated)/f control. In the instant disclosure, flux weakening iscompensated for by modifying the number of poles, called (V·p)/fcontrol, or decreasing the number of turns, V/(f·N) control. In either(V·p)/f control or V/(f·N) control V≦V_(rated).

Motor Operation

FIG. 5 shows speed-torque characteristics for the startingcharacteristic and characteristics 1-4 corresponding to four operatingregions described in the disclosure. The first operating regiondescribes operation including and between the starting characteristicand characteristic 1 (see FIG. 6). The second operating region describesoperation including and between characteristic 1 and characteristic 2(see FIG. 7). The third operating region describes operation includingand between characteristic 2 and characteristic 3 (see FIG. 8). Thefourth operating region describes operation including and betweencharacteristic 3 and characteristic 4 (see FIG. 9).

FIG. 6 shows 3 speed-torque characteristics between the startingcharacteristic and characteristic 1 of FIG. 5.

FIG. 7 shows speed-torque characteristic 1 and 2 of FIG. 5. The polenumber change does not generate sufficient torque to start the machinefrom a speed of zero. Instead a rated voltage V_(line-line-rated) isapplied and the frequency f is increased from a starting frequencyf_(start) to a base frequency f_(base). Preferably the startingfrequency f_(start) is not too low in order to avoid excessive currentand saturation of the magnetic components. When the frequency f isbetween f_(start) and f_(base), this operating region is referred to asthe starting region. In the starting region the speed spans 0 rpm to thebase speed n_(base).

FIG. 8 shows 4 speed-torque characteristics between characteristic 2 and3 of FIG. 5. The transitional speed-torque curves from the naturalcharacteristic 2 to the natural characteristic 3 are depicted in FIG. 8,and flux weakening (V/f) control will be employed, whereby the fluxdensities are reduced to below the rated value. FIG. 9 shows 3speed-torque characteristics between characteristic 3 and 4 of FIG. 5.From natural characteristic 3, at an increased (about rated) fluxdensity due to N=N₁=N₀/2, to natural characteristic 4 flux weakening(V/f) control with N=N₁ is performed. It is advisable to minimize thespeed range between characteristic 2 and characteristic 3, as depictedin FIG. 8. Note that, in FIG. 8, the origin of the speed n_(m)/n_(base)is not shown. The frequency 1.5·f_(base) has been chosen to clearlyillustrate the effect of the reduction of the number of turns, but anoptimization of the dynamics of the drive requires that 1.5·f_(base)should be less, say 1.3·f_(base).

FIG. 9 shows 3 speed-torque characteristics between characteristic 3 and4 of FIG. 5. At the natural characteristic 3 the number of series turnsN₀=N_(rated) will be reduced to N₁=N₀/2 by a relay (having normallyclosed, NC, and normally open, NO, contacts) or electronic switches,thus effectively restoring the flux density to its rated (or larger)value and increasing, therefore, the torque and the output power of themotor—due to the reduction of the series number of turns fromN₀=N_(rated) to N₁=N₀/2. This increase in torque is about proportionalto the ratio (N₀/N₁).

The current drawn at zero speed is I_(start)>I_(rated). As start-upoccurs the starting current will decrease due to the frequency and theassociated speed increase, and the motor reaches at the base frequencythe base speed.

An example of the above-described machine could comprise an electricmachine consisting of a pole-changing squirrel-cage induction machinewith an accompanied reduction of the number of stator turns per phase.In order to provide a starting torque of T_(start)/T_(rated)=11 (seeFIG. 5) the machine will start as a p₁=8-pole machine operating atf_(low)=f_(start) and a line-to-line voltage of aboutV_(line-to-line-rated) supplied by the inverter, resulting in anincrease of the flux density from say 0.55T to about 1.0T. Assume thatT_(rated)=T_(rated) _(—) _(4poles) corresponds to the rated torque ofthe p₂=4 pole machine, where T_(max) _(—) _(4poles)≈3·T_(rated) _(—)_(4poles), then the increase of the pole number by a factor of tworesults in T_(max) _(—) _(8poles)≈2·T_(max) _(—) _(4poles), and theincrease of the flux density during starting increases T_(max) _(—)_(8poles) by a factor of about (1.0/0.55)=1.82: that is,T_(start)=3·2·1.82=11·T_(rated) _(—) _(4poles) (see FIG. 5). In order toincrease the operating speed the frequency is increased fromf_(low)=f_(start) to f_(base). The speed should then increase toapproximately n_(rated)≈n_(base). At this point the machine must changefrom, for example, p₁=8-pole to p₂=4-pole operation. This change isinitiated by the controller. This change from p₁ to p₂-pole operationcauses the machine to speed up to about (p₁/p₂)n_(base). In order toincrease the speed from this point on the operating frequency isincreased from f_(base) to about (1.1-1.5)f_(base). This causes thespeed of the machine to increase to above (p₁/p₂)n_(base). In order toachieve further torque and speed increase (e.g., by a factor of 2), thenumber of series turns is reduced from N₀=N_(rated) to N₁=N₀/2.Thereafter, the operating frequency is increased to a multiple off_(base) which results in the maximum desirable speed at a relativelylarge torque (e.g., twice rated torque, T_(rated)) compared to thetorque generated when N₀=N_(rated).

FIGS. 10 and 11 show the time diagram for frequency changes and theresultant change in speed, respectively. The 8 to 4-pole change occursat 1.5 seconds and the N-reduction occurs between 3 and 3.5 seconds.

An example of the startup operation of the electric machine from 0 to6,000 rpm follows. Take the base speed of the drive to be n_(base)=750rpm. The frequency can be f_(base)=f_(rated)=50 Hz. The poleconfiguration p₁=8 poles. The rated torque T_(rated) _(—) _(4poles) isthat of a 4-pole machine (p₂ configuration) at the rated maximum fluxdensity B_(max)=0.55T and at the rated speed, n_(rated) _(—)_(4poles)=1,480 rpm. Thus, n_(rated) _(—) _(4poles) is approximately2·n_(base). This example assumes a three-phase induction machine with anoutput power of P_(rated) _(—) _(4poles)=500 W. This results in a ratedtorque of T_(rated) _(—) _(4poles) approximately equal to 2.7 Nm. Theline-to-line voltages of the three-phase machine are maintained constantthroughout the startup at V_(line-to-line)=√3·V_(phase)=√3·V_(DC)/(2√2).For a battery voltage—as used in the analysis and experimentalverification—V_(DC)=200 V one obtains the motor line-to-line voltageV_(line-to-line)=122.5 V_(rms). With these values the startup can bedescribed as follows:

-   1) At t=0 s the speed is 0 rpm (see FIGS. 10, 11). Stator current    frequency and voltage is f|_(0 s)=f_(startup)=25 Hz. Stator current    I_(max) _(—) _(start) ^(8-poles) is 15 A (see FIG. 29). Starting    torque T_(startup) ^(8-poles)=11·T_(rated) _(—) _(4poles) (see FIG.    5, operating point 2). The reduction of frequency from f_(rated)=50    Hz to f|_(0 s)=f_(startup)=25 Hz results in an increase of the flux    density (flux strengthening) by a factor of 2    B_(max)|_(0 s)=2·B_(max)=1.1 T, if saturation is neglected. The    torque is about proportional to the stator current and this starting    torque is reflected in FIG. 29: at p₂=4 pole and N_(0=N) _(rated)    steady-state operation I_(max) _(—) _(steady-state) ^(4-poles)=1.4 A    and at p₁=8 pole operation during start-up I_(max) _(—) _(startup)    ^(8-poles)=15 A. From this it follows that I_(max) _(—) _(startup)    ^(8-poles)/I_(max) _(—) _(steady-state) ^(4-poles)=15/1.4≈11 and is    not the same as that of the conventional armature (E/f) control (an    electric machine without pole switching).-   2) Startup from t=0-1.5 s (see FIGS. 10, 11) occurs in the p₁=8 pole    configuration. The motor increases its speed due to the increase of    the frequency from f_(startup)=25 Hz to f_(rated)=50 Hz and the flux    density decreases from B_(max) _(—) _(startup)=B_(max)|_(0 s)=1.1 T    to about B_(max)=B_(max) _(—) _(rated)|_(1.5 s)=0.55 T due to the    change of the pole number from p₁=8 to p₂=4 poles. This is reflected    in characteristic 1 of FIG. 5. The speed is n_(base)=750 rpm (see    FIG. 11) at f_(rated)=50 Hz, neglecting the slip.-   3) Pole switching occurs at t=1.5 s and the pole number is decreased    from p₁=8 to a p₂=4 at f_(rated)=50 Hz. The speed of the motor    doubles and is now n_(rated) _(—) _(4poles)≈(2·n_(base))≈1,480    rpm≈1,500 rpm at f_(rated)=50 Hz (FIG. 5, characteristic 2). At    about 1,480 rpm the rated torque T_(rated) _(—) _(4poles)≈2.7 Nm is    produced at B_(max)=0.55 T. The output power corresponds to its    rated value P_(rated) _(—) _(4poles)=500 W and is the same as that    of the conventional armature (E/f) control.-   4) The frequency increase during the period t=2-3.5 s from    f|_(2 s)=f_(rated)=50 Hz to f|_(3.5 s)=75 Hz corresponding to a    speed of n|_(3.5 s)≈(75/50)·1,500 rpm≈2,250 rpm results in field    weakening operation decreasing the flux density from B_(max) _(—)    _(rated)|_(2 s)=B_(max)=0.55 T to B_(max)|_(3.5 s)=(50/75)·0.55    T=0.37 T and reduces the torque from its rated value    T|_(2 s)=T_(rated) _(—) _(4poles) to T|_(3.5 s)=(50/75)·T_(rated)    _(—) _(4poles)=0.67·T_(rated) _(—) _(4poles). The output power at    t=3.5 s corresponds to the rated output power P_(rated) _(—)    _(4poles) and is the same as that of the conventional flux-weakening    (V/f) control.-   5) At t=3.5 s the number of series stator turns is reduced from    N₀=N_(rated) to N₁=N₀/2, this increases the flux density at    f|_(3.5 s)=75 Hz to B_(max)|_(3.5 s) _(—) _((N) ₀ _(/2))=0.73 T,    which means at t=3.5 s—after the number of turns have been reduced    by a factor of 2—the flux density is larger than the rated flux    density B_(max)|_(3.5 s) _(—) _((N) ₀ _(/2))=73 T, and the torque    increases to T|_(3.5 s)=1.33·T_(rated) ^(4-poles) at f|_(3.5 s)=75    Hz (see FIG. 5, characteristic 3). From FIG. 29 one gathers that the    current increases by a factor of about 5.0 A/1.4 A=3.57 which    supports the increase of the torque depending upon saturation by a    factor of 2 to 4. The output power at f|_(3.5 s)=75 Hz—after the    switching of the series turns has been completed—is at steady state    (increasing the flux density by a factor of 2), P_(out) ^(4-poles)    ^(—) ^((N) ⁰ ^(/2))|_(3.5 s)=2·P_(rated) _(—) _(4poles), which is    twice that of the conventional flux-weakening (V/f) control. One can    speak in this case of a compensation of flux weakening due to the    reduction of the number of turns from N₀ to (N₀/2).-   6) From t=3.5-4.5 s the frequency is increased from f|_(3.5 s)=75 Hz    to f|_(4.5 s)=100 Hz resulting in a speed of (2·n_(rated) _(—)    _(4poles))=(4·n_(base))=3,000 rpm. At t=4.5 s the torque is

${{T❘_{4.5\; s}} = {{\left( \frac{75}{100} \right) \cdot 1.33 \cdot T_{{{rated}\_}4{poles}}} \approx T_{{{rated}\_}4{poles}}}},$and the output power at t=4.5 s is P_(out) ^(4-poles) ^(—) ^((N) ⁰^(/2))|_(4.5 s)=2·P_(rated) _(—) _(4poles), which is twice that of theconventional flux-weakening (V/f) control. The flux density at t=4.5 sis B_(max)|_(4.5 s) _(—) _((N) ₀ _(/2))=0.55 T, which corresponds to therated flux density and is twice that of the conventional flux-weakening(V/f) control. This again shows that flux weakening is compensated forand the maximum flux density is restored.

-   7) Above t=5 s the frequency is further increased from    f|_(4.5 s)=100 Hz to f|_(5 s)=133 Hz and flux weakening sets in (see    FIG. 5, characteristic 4). The speed is    n|_(5 s)≈(133/100)·2·n_(rated) _(—) _(4poles)=(5.32·n_(base))=4,000    rpm at

${T❘_{5\; s}} = {{\left( \frac{100}{133} \right)T_{{{rated}\_}4{poles}}} \approx {0.75T_{{{rated}\_}4{poles}}}}$and B_(max)|_(5 s) _(—) _((N) ₀ _(/2))=(100/133)0.55 T=0.41 T, which istwice that of the conventional flux-weakening (V/f) control. As shown,the machine can be used as a generator charging the battery of the powersupply (FIG. 5, operating point 4) and depicted in FIG. 30 where V_(DC)_(—) _(rectifier)=200V. Note, the horizontal axis of the oscillogram isindicated by the arrow 2.

-   8) A further increase of the frequency at t=6 s from f|_(5 s)=133 Hz    to say f|_(6 s)=200 Hz (not shown in FIGS. 10 and 11) increases the    speed to n|_(6 s)≈(200/100)·2·n_(rated) _(—) _(4poles)=6,000 rpm    based on flux weakening. At the speed of n|_(6 s)=6,000 rpm the    maximum flux density will be B_(max) _(—) _(6000 rpm)=0.27 T and the    torque will be T_(6000 rpm) ^(4-poles)=0.5·T_(rated) _(—) _(4poles).    The output power at 8·n_(base)=4·n_(rated) ^(4-poles)=6,000 rpm will    be P_(out) _(—) _(6,000 rpm) ^(4-poles) ^(—) ^((N) ⁰    ^(/2))=(4·0.5·0.27/0.55)·P_(rated) _(—) _(4poles)≈P_(rated) _(—)    _(4poles). The speed range for (V·p/N·f) control, where the rated    output power is P_(rated—) _(4poles), is twice the speed range that    a conventional flux-weakening (V/f) control (see FIGS. 2, 3) can    achieve.

The starting operation of the motor is transient, that is, the motor hasto absorb a relatively large current, of multiples of the rated current(resulting in large losses), during a few seconds. This is acceptableprovided the cooling of the motor is sufficient (e.g., water cooling).Efficiency considerations are deemed to be unimportant during starting;however, maximum temperature rises are a concern and a thermal analysismust be performed. Harmonics are a concern as well, because the startingprocedure involves switching the number of poles from p₁ to p₂ andpulse-width-modulated (PWM) inverter operation, and both can result inharmonic torques.

Generator Operation

The alternator or regenerative braking operation occurs in quadrant IIof any of the characteristics 1 to 4 (see FIGS. 5-9). Once the machinehas gained about the base speed the only stringent requirement is anefficiency of between about 75-95%. At regenerative operation of thealternator the PWM inverter acts as a PWM rectifier providing excitationfor the generator if necessary, and delivering generator power to thebattery or the power source at a DC voltage of VDC.

Overall Machine Design

The machine can include phase belts, a controller, and a converter(inverter/rectifier). The poly-phase machine design is based on machinegeometric data and iron-core characteristics for the stator and rotor.As an example, a machine with 24 stator slots has been chosen. Othersuitable stator slot numbers (e.g., 36, 48, 54, 72 etc.) are possible. A3-phase stator winding has been chosen because 3-phase windings are veryfrequently used for drive applications. Other phase numbers are possibleas well, such as 2, 5, and higher number of phases. For this example,the pole numbers p₁=8 and p₂=4 have been chosen. Other pole numbercombinations are also feasible. All analytical and experimental resultsare based on a reduction of the number of series turns from N₀=N_(rated)to N₁=N_(rated)/2. Other reductions of the series turns, for instanceN₁=N_(rated)/3, are also possible.

Phase Belt Connections

The machine includes a plurality of phase belts, each wrapped around oneor more teeth of the stator. The phase belt inductance can be altered inone of two ways: by changing the connections between coils in a phasebelt from series to parallel, or vice versa; or by changing theconnections between phase belts in a given phase from series toparallel, or vice versa. Changing the connections between coils in aphase belt from series to parallel is equivalent to changing the numberof turns in a phase belt from a first number of turns N₀ to a second andlower number of turns N₁.

As described earlier, preferably two or more phase belts can form poles.The number of poles, like the number of turns (phase belt inductance),can also be electronically switched. The configuration for a firstnumber of poles can be defined as p₁, and the configuration for a secondnumber of poles can be defined as p₂. In an embodiment, the number ofpoles can be changed by switching the connections between phase beltsfrom series to parallel connections, or vise versa. There are multipleways to make such a change, but a particularly efficient way to do so(using the fewest switches) is to change the pole configurations fromdelta (Δ), to double wye (Y).

The reduction of the number of series turns from N₀=N_(rated) to N₁ canbe performed in several ways, three versions of which are describedbelow. For Version 2, experimental data are included.

FIG. 12 illustrates two 3-phase stator winding diagrams for anembodiment of the p₁-pole configuration, delta (Δ), and the p₂-poleconfiguration, double wye (Y).

FIG. 13 illustrates an embodiment of a p₁ pole configuration (N_(rated),8-pole, Δ). FIG. 14 illustrates an embodiment of a p₂ pole configuration(N_(rated), 4-pole, double Y). FIGS. 13, and 14 depict the windingdiagrams for p₁=8-pole at N₀, and p₂=4 pole at N₀=N_(rated)configurations, respectively. The p₁-pole configuration is implementedusing a Δ connection and the p₂-pole uses a double Y connection.

FIG. 15 illustrates terminal leads of phase belts for the embodimentsillustrated in FIGS. 13 and 14. These leads must then be connected tothe switching board (control system) so that they can be switched by acontroller. The controller may be connected to an application specificintegrated circuit (ASIC) representing the switches as described below.

FIG. 16 illustrates a 3-phase current diagram with a rotating time axis.FIG. 17 illustrates a plot of magneto-motive force (“MMF”) for the FIG.13 winding configuration. FIG. 18 illustrates a plot of MMF for the FIG.14 winding configuration. In addition, a position encoder may beattached to the shaft of the machine—or the speed can be obtained basedon computations—in order to implement closed-loop control; for open-loopcontrol such an encoder is not required.

FIG. 19 illustrates a first view of a first embodiment of a windingconfiguration. The 3-phase stator winding includes the switches requiredto achieve a reduction of the number of series turns from N₀=N_(rated)to N₁=N_(rated)/2 for Version 1.

FIG. 20 illustrates a second view of the winding configurationillustrated in FIG. 19. FIG. 20 illustrates the complete winding andswitch diagram for Version 1 including the switches for changing thenumber of poles and the number of turns.

Correspondingly, FIG. 21 illustrates a first view of a second embodimentof a winding configuration. The diagram of the 3-phase stator windingincludes the switches required to achieve a reduction of the number ofseries turns from N₀=N_(rated) to N₁=N_(rated)/2 for Version 2.

FIG. 22 illustrates a second view of the winding configurationillustrated in FIG. 21.

FIGS. 23, 24, and 25 depict the stator winding diagrams of the threephases including the switches required to achieve a reduction of thenumber of series turns from N₀=N_(rated) to N₁=N_(rated)/2 for Version3. FIG. 23 illustrates phase A of a third embodiment of a windingconfiguration. FIG. 24 illustrates phase B of the third embodiment of awinding configuration. FIG. 25 illustrates phase C of the thirdembodiment of a winding configuration.

In order to implement the switches, relay or electronic switches can beemployed. For Version 2 a total of 10 relays are used: 7 of them have 3NO (normally open) and 3 NC (normally closed) contacts and the remaining3 relays have 2 NO and 4 NC contacts. Note, that not all contacts areused: for Version 2, Table 1 lists the number and types of contacts forthe implementation of the winding changes, that is, switchover from p₁to p₂ poles and the reduction of the number of series turns per phasefrom N₀=N_(rated) to N₁=N_(rated)/2. Switch group refers to the numbershown next to each switch in FIG. 22 representing Version 2. Table 1indicates that for Version 2 21 NO and 30 NC contacts are required.Similar considerations apply to Versions 1 and 3. Version 2 is the mostgeneral and flexible approach, but it requires the most switches.Version 3 requires the least amount of switches: in this latter case theseries connection of (two) phase belts of each phase is reconfiguredfrom series connection to parallel connection.

TABLE 1 Winding switches required for Version 2, FIG. 22 p₁ = 8 poles,p₂ = 4 poles, p₂ = 4-poles, Switch Group N₀ N₀ N₁ = N₀/2 1 (6 NC,normally closed) on off off 2 (9 NO, normally open) off on on 3 (24 NC,normally closed) off off on 4 (12 NO, normally open) on on off

Converter (Inverter/Rectifier) Design

The converter may fulfill at least the following two functions: (1)inverter operation during starting acceleration (motoring), and (2)rectifier operation during regenerative braking as the poly-phasemachine functions as an alternator feeding DC power to the battery orpower source.

In addition, the converter may provide for both (motoring, generation)operating regimes reactive power to the poly-phase machine in order tosupply its magnetizing current so that the appropriate flux can beestablished within the machine. Alternatively, the induction generatorcan be excited by a capacitor bank.

The required specifications of the converter operated as an inverter areas follows:

Input DC voltage: V_(DC)

Frequency range: f_(low)≦f_(base)≦f_(high)

Current at low frequency f_(low): multiple times the rated current perphase (maximum current).

Output voltage: as high as possible for given input voltage V_(DC)

Power factor: lagging power factor cos(φ) from 0 to 1.0.

Maximum current limitation to limit current spikes during switching ofnumber of turns N.

The required specification of the converter operated as a rectifier isas follows:

Output voltage: V_(DC) at multiple times I_(alternator-rated).

Controller Design

The controller in accordance with this disclosure preferably may bedesigned so that a successful start-up from zero speed to multiple timesthe base speed can occur during a few seconds. The controller may beconfigured to do the following at a constant output voltage,V_(line-line-rated), of the inverter: (1) change the frequency of theinverter from f_(low) to f_(base) when operating at speeds below (at p₁and N_(rated)) and including characteristic 1 at N_(rated); (2) providethe switching signals for switching the stator phase belts from ap₁-pole configuration to a p₂-pole configuration at f=f_(base) and ratedV_(line-line-rated) (or reduced) output voltage of the inverter. As aresult the motor operates on natural characteristic 2 at f=f_(base) atN_(rated); (3) initiate the switching signal so that the number ofseries turns per phase belts can be reduced and change the frequency atconstant output voltage of the inverter V_(line-line-rated) fromf_(base) to f>f_(base) resulting in (V_(rated)/[f·(N₁/N_(rated))])control so that natural characteristic 3 can be reached at f>f_(base);(4) change the frequency at constant output voltage of the inverterV_(line-line-rated) from f>f_(base) to f being a multiple of f_(base),resulting in (V_(rated)/f·N₁) control so that natural characteristic 4can be reached at f being a multiple of f_(base); (5) provide gatingsignals so that the PWM inverter can be used as a PWM rectifier feedingcurrent into the battery or power source at V_(DC) during regenerativebraking, and at the same time provide the magnetizing current of the(e.g., induction) machine. Alternatively, the induction generator can beexcited by a capacitor bank. To limit the charging current to thebattery or power source, the output voltage of the alternator must beadjustable (e.g., either increased or decreased) so that thebattery/power source lifetime will not be reduced by charging with atoo-large current; (6) either provide switching signals for currentsnubbers so that the switching spikes of the currents can be limited andthe inverter rating must therefore not be increased to accommodate thespikes or alternatively limit the maximum inverter output current sothat no current spikes can occur during switching of the number ofturns.

Experimental Verification

In most electric machines the torque is about proportional to the squareof the rotor current I. As an experimental verification of the reductionof the number of series turns N_(rated), the starting torque and theline current were measured. At low saturation there exists about asquare relationship between starting torque and the starting current,that is, T_(start)≈constant₁·I_(start) ², however, at high saturationthis relationship is about linear, that isT_(start)≈constant₂·I_(start).

FIG. 26 is an oscilloscope readout showing current versus time, andcreated by the winding configuration illustrated in FIGS. 21 and 22. Thestart-up currents from zero to maximum speed are depicted for differentconditions in FIGS. 27 to 29. FIG. 27 is an expanded view of the startupregion of the oscilloscope readout of FIG. 26. In this region there isno pole changing or number of turns changing. It merely shows the hightorque that can be achieved by using a high number of poles duringstartup (at low speeds).

FIG. 28 is an oscilloscope readout showing the startup region, and theswitch from N₀ to N₁ occurring many seconds after the switch from p₁ top₂. At about 3.5 seconds a transient current spike shows the timing ofthe pole configuration change from p₁ to p₂. About 4.3 seconds after thepole change, the number of turns is decreased from N₀ to N₁. During this4.3 second region, torque is decreasing since the machine suffers fromflux weakening or (V/f) control in this region (after the pole switchbut before the number of turns decrease). To avoid this flux weakeningregion, the time between pole switching and number of turns decrease canbe minimized.

FIG. 29 is an oscilloscope readout showing the startup region, and theswitch from N₀ to N₁ occurring around one second after the switch fromp₁ to p₂. In this example, the time between the pole switch and thenumber of turns decrease is much shorter than that illustrated in FIG.28. As a result, the torque is returned to a higher value quicker—fluxweakening comes into play for a shorter period of time. The reduction ofthe number of turns occurs 1.0 second after the switchover from p₁ top₂; in this case the current spike due to the reduction of the number ofturns is larger. Ideally the number of turns would be decreased rightafter the pole configuration change in order to avoid any flux weakeningregion. However, it was found that if the timing between the pole changeand decrease in the number of turns was less than about one second, themachine would saturate, thus decreasing the machine's efficiency. Thereis also the potential for permanent damage. Thus, for now the minimumtiming between pole change and the decrease in the number of windings isaround one second, but this is not a limiting factor. In the futuremeans will likely be found to allow a shorter (in terms of seconds) fluxweakening region between the pole change and decrease in the number ofturns, such that saturation is avoided.

From FIG. 29 one concludes that the starting current (or torque)corresponds to about 3 units while the maximum current (or torque) atmaximum speed with N₁=N_(rated)/2 corresponds to 1 unit. Comparing thisto the operation with p₂ and N_(rated)—where the maximum current (ortorque) at maximum speed is 0.25 units—one finds that the reduction ofthe number of turns by a factor of 2 increases the maximum current (ortorque) by a factor of at least 3, assuming a proportionality betweentorque T and current I. To minimize the inverter power/current ratingthe current spikes can be either suppressed by current snubbers or themaximum inverter output current can be limited via inverter design. Thissuppression is not shown in the oscillograms of FIGS. 26-29.

FIG. 30 is an oscilloscope readout showing voltage versus time for agenerator (upper trace) and a rectifier (lower trace), and created bythe winding configuration illustrated in FIGS. 21 and 22.

Advantages of the Disclosure

Throughout this specification, exemplary embodiments have been describedand illustrated. The two key concepts to achieve this performance(increase in speed and torque) are: (1) switching (using relays orelectronic switches) of the stator number of poles from p₁ to p₂, and(2) switching (using relays or electronic switches) of the number ofseries turns in each phase belt. These two concepts can be eitherseparately or jointly implemented in a variable-speed drive inaccordance with the present disclosure. In accordance with the presentdisclosure, many improvements in machine construction and operation maybe realized.

These may include the possibility of designing a poly-phase electricmachine capable of delivering starting torque within the range(5-11)·T_(rated). The lower range (5·T_(rated)) can be obtained, forexample, with standard-type (e.g., squirrel-cage induction) machines andthe upper range (11·T_(rated)) is obtained with machine designs havingreduced stator resistance and leakage inductances. In the case of aninduction machine a double-squirrel cage rotor winding may improve thedynamic performance during switching of the number of turns.

(V_(rated)/f) control, at steady-state and while above the base speed,generates no more than rated output power (or torque). In contrast,(V_(rated)/f·N) control, at steady-state and while above the base speed,generates greater-than-rated output power (e.g., twice the output powerat two times the rated speed). This is accomplished via reducing thenumber of series turns N in each phase belt. (V_(rated)/f) controlvariable-speed drives, at steady-state, can generate a power density perunit weight of 3 kW/(kg-force). In contrast, (V_(rated)/f·N) control, atsteady-state, can generate a power density per unit weight of up to 5-6kW/(kg-force).

While with (V_(rated)/f) control the maximum obtainable speed is about 3times the base speed, with (V_(rated)·p/f·N) control about 4 times therated speed, and about 6-8 times the base speed can be obtained. Thisspeed increase may make mechanical gears obsolete and thus reducevehicle weight, cost of production, and cost of maintenance.

Higher torque at lower speed is obtained through increasing flux-densityabove its rated value. This process is also known as flux strengthening.Increased torque at higher speed is obtained through compensation offlux weakening. Increased torque at high speed can improve the poweroutput by a factor of two and lead to the power density of 5-6kW/(kg-force).

The (V_(rated)·p/f·N) control appears to be suitable for hybrid/electricpropulsion applications because the reduction in the number of turns Nfor each phase belt is equivalent to an increase of the applied voltageV_(rated) without actually increasing the voltage of the power supply(e.g., battery, DC capacitors). This is advantageous for vehicleapplications where the battery voltage is limited to, say 300V_(DC)because of safety considerations. For example, although the electricmachine is supplied with 300V_(DC) only, it behaves like a machine fedby 600V_(DC), resulting in larger (e.g., two times) output power perunit of weight measured in kW/(kg-force).

Despite vehicle battery voltages often being limited to 300 V_(DC) (forsafety), the (V·p/N·f) control leads to a motor line-to-line voltage ofV_(line-to-line) equal to 184 V_(rms). A speed range of 0 to 6,000 rpmcan be achieved via the resulting output power/torque characteristics ofthe machine under (V·p/N·f) control. In contrast, conventional (V/f)control can only achieve this speed range if a V_(line-to-line) of 368V_(rms) is used. This voltage requires a DC-DC converter to convert theconventional 300 V_(DC) battery voltage into a 600 V_(DC) voltage(required to reach the V_(line-to-line) of 368 V_(rms)). The converterincreases total weight and increases the losses by at least about 4%.The mechanical gears required to produce the required starting torquefurther increase the drive train weight.

Inverter switches (conducting in reverse direction) or a separate diodebridge together with AC capacitors can be employed to operate theelectric (induction) machine as a generator supplying regenerated energyto battery or DC capacitors. Thus the (e.g., induction) machineoperating as a generator can either be excited via the PWM inverter fromthe battery or via AC capacitors.

For automobiles with internal combustion engine only, the starter andthe generator can be integrated in one machine. The application of(V/f·N) control—where V≦V_(rated)—to wind power plants permits asimplification of the blade-pitch control of the wind turbine because asthe speed of the wind turbine increases the torque of the generator canbe increased, and the turbine can be operated at any speed—except themaximum speed where feathering or shut down occurs—under stalledconditions.

To summarize, this disclosure allows (V·p/f·N) control over an electricmachine capable of the following: (1) producing programmabletorque-speed characteristics with increased torques below and aboverated speed (5-11 times the rated torque, and at least twice the ratedtorque, respectively) through flux reinforcement and compensation offlux weakening; (2) generating a range of 0 to 6-8 times base speed (atleast twice that of state-of-the-art drives); (3) generating at leasttwice the output power (5-6 kW/kg-force) as state-of-the-art drives; (4)using half the battery voltage (300V), which is much safer than the 600Vof (V/f) control for the speed range of (2); and (5) resulting in higherefficiency and less weight because there is no need for a DC-DC boostconverter or mechanical gears.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. For example, whilethe instant disclosure has been described in terms of the p₁ statorwinding being connected in Δ and the p₂ pole stator winding in double Y,other configurations are also possible. Where p₁ is configured in Δ andthe p₂ in double Y, (P_(p1-design))/(P_(p2-design)) is less than 1 andits torque ratio is larger than 1. However, for hybrid/electric driveswhere a large torque is preferred at low speed, the p₁ pole statorwinding may be connected in double Y and the p₂ pole stator winding maybe connected in Δ. In that case the power ratio,(P_(p1-design))/(P_(p2-design)), is about 1 and its torque ratio isabout 2. Alternatively, a configuration where torque or output powerincreases with angular velocity, ω_(m), suitable for wind power plants,the p₁ pole stator winding may be connected in Y and the p₂ pole statorwinding may be connected in double Y. This results in a power ratio(P_(p1-design))/(P_(p2-design)), which is about one half, and which hasa torque ratio about one. It should be expressly understood that suchmodifications and adaptations are within the spirit and scope of thepresent invention.

1. A multiphase inductance-changing and pole-changing electric machineoperable at above rated parameters comprising: a stator having aplurality of phase belts, wherein each phase belt comprises two or morecoils, and wherein each of the two or more coils in a phase belt areinitially connected in series; a rotor driven by currents in the stator;and a control system connected to the plurality of phase belts andconfigured to: (a) apply an alternating current to the plurality ofphase belts at a first time and at a frequency equal to a firstfrequency, wherein the first frequency is below a rated frequency; (b)increase the frequency to a second frequency during a time periodspanning the first time and a second time; (c) decrease a number ofpoles at a third time; (d) increase the frequency to a third frequencyduring a time period spanning a fourth time and a fifth time; (e)decrease the inductance of the plurality of phase belts at a sixth time;and (f) increase the frequency to a fourth frequency during a timeperiod spanning a seventh time and an eighth time.
 2. The machine ofclaim 1, wherein to decrease the number of poles, the series connectionsbetween phase belts are switched to parallel connections.
 3. The machineof claim 1, wherein the time period spanning the fourth and fifth timeperiods is minimized without saturating the machine.
 4. The machine ofclaim 1, wherein to decrease the inductance of the plurality of phasebelts, the series connections between the two or more coils in the phasebelts are switched to parallel connections.
 5. The machine of claim 1,wherein to decrease the inductance of the plurality of phase belts, theseries connections between the phase belts in a phase are switched toparallel connections.
 6. The machine of claim 1, wherein the phase beltsare initially connected in a delta configuration, and wherein todecrease the number of poles, the delta configuration is switched to adouble wye configuration.
 7. The machine of claim 1, wherein the phasebelts are initially connected in a double wye configuration, and whereinto decrease the number of poles, the double wye configuration isswitched to a delta configuration.
 8. The machine of claim 1, whereinthe phase belts are initially connected in a wye configuration, andwherein to decrease the number of poles, the wye configuration isswitched to a double wye configuration.
 9. The machine according toclaim 1, wherein the second frequency equals the rated frequency. 10.The machine according to claim 1, wherein the second time equals thethird time.
 11. The machine according to claim 1, wherein the fifth timeequals the sixth time.
 12. A multiphase inductance-changing andpole-changing electric machine operable at above rated parameterscomprising: a stator having a plurality of phase belts, wherein eachphase belt comprises two or more coils, and wherein each of the two ormore coils in a phase belt are initially connected in series; a rotordriven by currents in the stator; and a control system connected to theplurality of phase belts and configured to: (a) increase frequency of analternating current applied to the plurality of phase belts during afirst time period; (b) decrease a number of poles in the stator at asecond time; (c) further increase the frequency of the alternatingcurrent applied to the plurality of phase belts during a second timeperiod; (d) decrease an inductance of the plurality of phase belts at athird time; and (e) further increase the frequency of the alternatingcurrent applied to the plurality of phase belts during a third timeperiod.
 13. The machine of claim 12, wherein the second time periodfollows the second time.
 14. The machine of claim 12, wherein the thirdtime period follows the third time.
 15. A method of operating analternating current machine at above rated parameters, the methodcomprising: (a) increasing frequency of an alternating current appliedto a plurality of stator phase belts during a first time period; (b)decreasing the number of poles in the stator at a second time; (c)further increasing the frequency of the alternating current applied tothe plurality of stator phase belts during a second time period; (d)decreasing the inductance of the plurality of stator phase belts at athird time; and (e) further increasing the frequency of the alternatingcurrent applied to the plurality of stator phase belts during a thirdtime period.
 16. The method of claim 15, wherein the second time periodfollows the second time.
 17. The method of claim 15, wherein the thirdtime period follows the third time.